Communication systems generally require partitioning of the electromagnetic frequency spectrum. Communication transceiver devices therefore must be capable of high frequency selectivity, i.e., capable of selecting a given frequency band while rejecting all others. Frequency-selective devices, such as filters, oscillators and mixers are therefore some of the most important components within a transceiver and the quality of the devices generally dictate the overall architecture of a given transceiver.
In wireless radio frequency (RF) devices, resonators are generally used for signal filtering and generation purposes. The current state of the art typically is the use of discrete crystals to make the resonators. To miniaturize devices, MEMS resonators have been contemplated. One type of MEMS resonator is a film bulk acoustic resonator (FBAR). A FBAR device has many advantages over prior art resonators such as low insertion loss at high frequencies, and lower thermal mass due to their compact size.
The resonance frequency of a FBAR device is determined by its thickness, which must be precisely controlled in order to have the desired filter response, such as exact central frequency and pass bandwidth. In a typical (FBAR) device, the resonance frequency after processing is usually different from the targeted value due to processing variation. For discrete crystal resonators as mentioned above, such resonance frequency error may be corrected using laser trimming technology, for example, in which a laser is directed towards the resonator and either removes or adds material to the resonator, thereby “tuning” the resonating frequency of the resonator to the desired targeted frequency. However, because MEMS resonators (particularly high frequency MEMS resonators) are generally much smaller in size than their crystal counterparts, traditional laser trimming technology is not a viable alternative. Accordingly, what is needed are techniques to modify the resonance frequency of a MEMS resonator.